Calories and joules
The thermochemical calorie is exactly 4.184 joules, about the energy to warm 1 g of water by 1 °C. Chemistry uses small calories. Food "Calories" are kilocalories, a thousand times larger.
The joule is the SI unit of energy: the work done by a force of one newton over one meter, or one watt for one second. Physics, engineering and EU food labels (as kJ) use joules.
Worked example
Convert 500 cal: 500 × 4.184 = 2,092, so 500 calories is 2,092 joules. Likewise 100 cal = 418.4 J.
Estimating in your head
Multiply by 4.2 for a quick figure; it is off by about 0.4%. For 500 cal that gives 2,100 J, against 2,092 exactly.
Going the other way
To convert joules to calories, divide by 4.184 (or multiply by 0.239005736138). The swap button between the boxes opens that page.
How this works
J = cal × 4.184
1 cal = 4.184 J and 1 J = 1 J, both by definition, so the factor is their ratio: 4.184 exactly. Results drop binary floating-point noise beyond 15 significant digits and display at the precision you choose (6 significant figures by default). Copy result copies the full value.
Frequently asked
How many joules are in a calorie?
1 calorie = 4.184 joules, exactly. The value follows from the unit definitions, so it never changes.
How do I convert calories to joules?
Multiply by 4.184. For example, 100 cal × 4.184 = 418.4 J.
What is 1,000 cal in joules?
1,000 cal = 4,184 J. The table next to the converter lists more common values.
Is the result rounded?
Only on screen. The page computes with the exact definitions and removes floating-point noise past 15 significant digits; pick 3 to 12 significant figures, or use Copy result for the full value.
Sources
- NIST SP 811 Appendix B.8 — Factors for units listed alphabetically
- BIPM — The International System of Units (SI Brochure), 9th edition
- Wikipedia — Calorie
Reviewed Oct 17, 2026 · test vectors fixtures/units/energy/calories-to-joules/